A cryogenic air separation unit (ASU) is typically designed, constructed and operated to meet the base-load product slate demands/requirements for one or more end-user customers and optionally the local or merchant liquid product market demands. Product slate requirements typically include a target volume of high pressure gaseous oxygen, as well as other primary co-products such as gaseous nitrogen, liquid oxygen, liquid nitrogen, and/or liquid argon. The air separation unit is typically designed and operated based, in part, on the selected design conditions, including the typical day ambient conditions as well as the available utility/power supply costs and conditions.
Although present in air in very small quantities, rare gases such as neon, xenon, krypton and helium are capable of being extracted from a cryogenic air separation unit by means of a rare gas recovery system that produces a crude stream containing the targeted rare gases. Because of the low concentration of the rare gases in air, the recovery of these rare gas co-products is typically not designed into product slate requirements of the air separation unit and, therefore the rare gas recovery systems are often not fully integrated into the air separation unit.
For example, neon may be recovered during the cryogenic distillation of air by passing a neon-containing stream from a cryogenic air separation unit through a stand-alone neon purification train, which may include a non-condensable stripping column and a non-cryogenic pressure swing adsorption system to produce a crude neon product (See e.g. U.S. Pat. No. 5,100,446). The crude neon product is then passed to a neon refinery where the crude neon stream is processed by removing helium and hydrogen to produce a refined neon product. For example, the neon recovery system disclosed in U.S. Pat. No. 5,100,446 has only moderate neon recovery about 80% because the neon containing stream that feeds to downstream neon stripping column is from non-condensable vent stream from main condenser-reboiler.
Moreover, where the rare gas recovery systems are coupled or partially integrated into the air separation unit as shown in U.S. Pat. Nos. 5,167,125 and 7,299,656; the rare gas recovery systems often adversely impact the design and operation of the air separation unit with respect to the production of the other components of air because a relatively large flow of nitrogen from the air separation unit must be taken in order to produce a crude neon vapor stream. For example. the low pressure (i.e. about 20 psia) neon recovery system disclosed in U.S. Pat. No. 7,299,656 has a very low neon concentration in the crude neon vapor stream of only about 1300 ppm, and therefore the crude neon product taken out from air separation unit is as high as almost 4% of liquid nitrogen reflux that is fed to the lower pressure column. Such significant loss of liquid flow that would be otherwise used as liquid reflux in the lower pressure column adversely impacts the separation and recovery of other product slates. In addition, such low neon concentration (i.e. 1333 ppm) crude product will cause higher associated operation cost in terms of compression power and liquid nitrogen usage to produce the final refined neon product. See also United States Patent Application Publication NO. 2010/0221168 which discloses a neon recovery system. The concentration of neon in the crude neon vapor stream is also relatively low at about 5.8%, and the recovery system is only applicable to the air separation unit with dirty shelf liquid withdraw where the liquid reflux fed to the lower pressure column is taken from the intermediate location of the higher pressure column.
What is needed is a rare gas or non-condensable gas recovery system that can produce a crude neon vapor stream that contains greater than about 50% mole fraction of neon and demonstrate an overall neon recovery of greater than about 95% with minimal liquid nitrogen consumption and minimal impact on the argon recovery in the air separation unit. In addition, as none of the above-described prior art neon recovery systems have the ability to easily and efficiently co-produce xenon and krypton, further needs include a rare gas recovery system that has overall neon recovery of greater than about 95% and can co-produce a crude neon vapor stream that contains greater than about 50% mole fraction of neon and greater than about 50% mole fraction of helium as well as produce commercial quantities of xenon and krypton.